1. Field of the Invention
The present invention relates to a method of producing a solid oxide fuel cell.
2. Description of Related Art
FIG. 3 shows the general structure of a planar solid oxide fuel cell. A fuel electrode 21, a solid electrolyte 22 and an air electrode 23 are laminated to form a generating section 24, which is the minimum unit of a fuel cell. In the generating section 24, hydrogen and air (oxygen) which are supplied from outside react to each other on the electrodes, and thereby, electrical energy is generated.
Generally, in order to obtain a high voltage, a plurality of generating sections 24 are electrically connected in series, and actually, are piled with interconnectors 25 thereamong. The interconnectors 25 prevent hydrogen supplied to the fuel electrodes 21 and air (oxygen) supplied to the air electrodes 23 from mixing together and also function as electronic conductors to electrically connect the generating sections 24 in series. A generating section 24 and two interconnectors 25 which sandwich the generating section 24 are called a cell.
Each interconnector 25 has grooves 26 on both sides, and the grooves 26 on one side and those on the other side extend in mutually perpendicular directions. The grooves 26 which face the fuel electrode 21 are a passage for hydrogen, and the grooves 26 which face the air electrode 23 are a passage for air (oxygen). The fuel electrode 21 and the air electrode 23 are smaller than the solid electrolyte 22. In FIG. 3, the fuel electrode 21 and the air electrode 23 are laid over the portions of the solid electrolyte 22 enclosed with the dashed lines. The portions of the solid electrolyte 22 outside the dashed lines have a width equal to the width of ungrooved edges of the interconnectors 25. Accordingly, the ungrooved edges of the interconnectors 25 are to be connected and the solid electrolyte 22, and the grooved portions on both sides thereof are to be connected to the fuel electrode 21 and the air electrode 23 respectively. For the connections between the grooved portions of the interconnectors 25 and the fuel electrode 21 and the &it electrode 23, a conductive material is used as the joining agent. 0n the other hand, for the connections between the ungrooved edges of the interconnectors 25 and the solid electrolyte 22, a material with a gas sealing function is used as the joining agent in order to seal the inside of the solid oxide fuel cell and to prevent hydrogen and air (oxygen) from mixing together.
The connections between the ungrooved edges of the interconnectors 25 and the solid electrolyte 22 have been conventionally carried out as follows. The gas sealing portions (ungrooved edges) 27 of the interconnectors 25 are coated with a glass-containing material in a slurry state and are put into contact with the generating section 24. Then, while a weight is being applied to the interconnectors 25 and the generating section 24, the glass-containing material is heated and fused. Thereafter, when the glass-containing material is cooled and hardened, the gas sealing portions 27 are connected to the solid electrolyte 22 of the generating section 24.
In the method, since a glass-containing material in a slurry state is used, it is difficult to form glass layers with an even thickness, and voids are likely to occur on the interfaces. In other words, there is a high possibility that the fuel cell does not have a sufficient airtightness. Thus, this method has a disadvantage that many defective products are made in this step of joining interconnectors and generating sections. During operation of the fuel cell at a temperature of about 1000.degree. C., the glass-containing material melts into liquid and therefore functions as a sealant effectively. However, as the fuel cell is operated longer, the glass-containing material is flowing out and losing the function as a sealant.
In the light of the problem, Japanese Patent Laid Open Publication No. 3-67466 disclosed that a material in a slurry state which is a mixture of a liquid glass-containing material and a ceramic filler is used.
In this method, when a weight is applied to the interconnectors 25 and the generating section 24, the slurry may come out of the connecting portions. Thereby, the connecting portions may have an uneven thickness, and the generating section 24 and the interconnectors 25 may be contaminated. Also, there is a possibility that a fire-resisting article which is used as the weight can adhere to the generating section 24 because of the slurry which comes out of the connecting portions. In this case, in separating the weight from the generating section 24, the generating section 24 may be damaged by the glass contained in the slurry.
For even coating of a material in a slurry state, screen printing is conventionally adopted. However, screen printing is available only for flat surfaces and requires a screen formed pattern in conformity with the configuration of the surface. As far as joining of the interconnectors 25 to the generating section 24 is concerned, considering that two different materials must be coated on neighboring portions (the gas sealing portions 27 of the interconnectors 25 must be coated with a glass-containing material, while the grooved portions must be coated with a conductive material), coating by screen printing is very difficult.